Method and apparatus for sodium recovery in a semi-chemical pulping operation

ABSTRACT

A method and apparatus for combusting black liquor generated by semi-chemical and other pulping processes, using a draft airflow furnace such as a rotating kiln, and a lower end mixing nozzle for black liquor and combustion air. A combustion agent is added to the black liquor to support ignition and permit a closely controllable combustion plume or flame adjacent the nozzle. Resulting combustion is complete and well below government environmental air emission standards. Chemical recovery is efficient, clean, and free from products of incomplete combustion.

[0001] This application claims priority for all purposes to pending U.S. application ser. No. 60/344,175, filed Dec. 27, 2001.

FIELD OF INVENTION

[0002] This invention relates to the recovery of sodium constituents from spent cooking liquor in a Semi-Chem pulp digesting process, and in particular to methods and apparatus for burning black liquor in a rotating furnace.

BACKGROUND

[0003] Paper and cardboard is manufactures using wood fiber. Wood fiber is derived from wood chips which are pulped using a variety of methods (such as Kraft, Sulfite, Mechanical, Semi-Chemical, etc.) as are well known in the industry. The pulp is wood fiber (cellulose) without the lignin binder.

[0004] The removal of lignin is usually performed in digesters using heat, pressure, and chemicals. The resulting slurry leaving the digesters is separated into two streams, pulp and weak black liquor. The pulp consists primarily of water and wood fiber. Pulp is washed in brown stock washers, and then used for paper or board manufacture. The weak black liquor (WBL) consists primarily of water, the chemicals used in the digester, and the lignin extract. The WBL is concentrated by multi-effect evaporators to form strong black liquor (SBL) and then burned in a boiler, kiln, or other combustion devise to generate steam.

[0005] The lignin portion of the SBL is a hydrocarbon, and burns like fuel oil to create heat and products of combustion (water and carbon dioxide). The chemical portion of the SBL is not combustible, and forms smelt, which dissolved in a smelt tank, and returned to the digesters for further use in pulping. In some processes (Kraft, Sulfite, and others) the smelt needs to be chemically treated and processed prior to return to the digester.

[0006] Products in the paperboard area are divided between containerboard and boxboard. Containerboard is the material from which corrugated containers are made. It comprises two types of paperboard: linerboard and corrugated medium, both of which are needed to make corrugated containers. Linerboard is the material used on the outside of corrugated boxes and generally utilizes softwood or coniferous tree fibers. It is made in a wide variety of basis weights, from both virgin and recycled fibers. Corrugating medium is the fluting material comprising the middle portion of corrugated containers. It too is made in a variety of basis weights, from both semi chemical pulp and recycled fiber, generally utilizing hardwood or deciduous tree fibers.

[0007] A significant portion of the corrugating medium is produced from semi-chemical processed pulp, a process using pulp liquor or “black liquor” as it is called. In order to make the operation more efficient and cost effective, it has long been a goal to recover the energy and the sodium from spent pulping liquor. Rotary furnaces have been used for several years for this purpose. Recovery processes include the Tomlinson smelter-furnace, the Zimmerman wet oxidation process, and the Direct Alkali Recovery System (DARS), where spent liquor is burned in the presence of ferric oxide and the sodium ferrite formed reacted with water to give sodium hydroxide and ferric oxide which is recycled.

[0008] In the mid 1970's a neutral sulphite-chemical recovery process (Sonoco process) came into limited use; there were three systems known to the Applicant to have been installed, including its own. The mode of operation was similar to a lime kiln. In a typical installation, a pelletized mixture of recovered ground ash, strong spent liquor, Alumina, and sometime sulphur, was fed into the uphill end of the rotating kiln, a 200 foot long by 12 foot diameter rotating combustion chamber or furnace. The furnace was set at a slight incline, so that materials migrated down the length

[0009] As the pellets traveled down the length of the kiln, they were heated to borderline smelt using an oil fired burner. At this point the remainder of the water was evaporated, the lignin was incinerated, and the sulphur dioxide was released into the exit gas stream. What remained were pellets of soda-ash and alumina which exited the downhill end and entered a rotary cooler. There the product was further rolled into “marbles” and cooled. It was the hammermilled and conveyed into an underground ash bin.

[0010] From there the ash was metered in two directions. Some was conveyed to a dissolving tank, mixed with combined condensate, and transferred to the absorption tower where dissolved ash and sulphur dioxide from the kiln reacted to form recovered liquor. The amount of ash used was determined by pH. This liquor was then clarified and the Alumina was separated using a belt filter for re-use in the pelletizer. The remainder was ground through pulverizers into the ground ash bin for use in the pelletizer as a base for making new pellets.

[0011] Because of numerous mechanical and process problems, the Sonoco recovery system as installed and used by this Applicant, was abandoned during 1979, in favor of burning the black liquor directly in the rotating kiln. Kiln operating parameters were established based on experience. Parameters were varied, attempting to obtain a more efficient burn and better conformance to government emissions requirements. The ultimate objective is to operate a closed chemical system, and to utilize the heat from the burning liquor as well, for steam production. However, the recovery process as presently practiced has not achieved that objective and is also extremely destructive of the refractory and various internal kiln structures such as the nose ring, duct work, firing hood, and so on. Over the course of time, the kiln has required a great deal of repair expense. The cost per linear foot for refractory installation is about US$3000.00, when done on a scheduled basis, and significantly more when done on a non-scheduled basis.

[0012] In the United States, the regulations regarding air and water quality standards for pulp and paper companies grew more stringent under the Cluster Rule, which the EPA (Environmental Protection Agency) published in the Federal Register in April 1998. Compliance deadlines for the multi-phase rule are staggered; the first pollution control plans were required by Jun. 15, 1999. Additional requirements particularly affecting sulphur based containerboard production operations and the handling of the black liquor take effect in 2002.

[0013] In particular, the Applicant's operation uses a sodium based, semi-chemical pulping process. A weak solution of sodium hydroxide and/or soda ash is mixed with wood chips and digested in a continuous digester using steam heat and pressure. The WBL is concentrated in multi-effect evaporators, and then burned in the rotating kiln. The combustion gas passes through a waste heat boiler to generate steam for the process, and then exits through a wet ionizing scrubber for dust removal. The smelt rolls down to the lower end of the kiln and enters a smelt tank, where it is dissolved to form make-up liquor for the digestion process. The make-up liquor consists primarily of water, sodium hydroxide, and soda ash.

[0014] The recovery of black liquor to produce heat and return spent chemistry to the process is essential for economic and environmental purposes. Heat produced from lignin combustion replaces an equivalent amount of fuel oil or natural gas. Smelt recovery replaces an equivalent amount of fresh chemicals. Disposal of the lignin and spent chemicals is environmentally prohibited and undesirable.

[0015] The recovery of heat and chemicals from the Applicant's pulping process is highly regulated under recent environmental standards. The United States Environmental Protection Agency (USEPA) has established the Cluster Rule (MACT I, MACT II, and MACT III) which closely limits the emission of hazardous air pollutants (HAPs) from the paper and pulping operations. The combustion of black liquor is regulated by the MACT II standard. States have also established new emission limits (in addition to those by the USEPA) for toxic air pollutants. Wastewater and solid waste standards also scrutinize toxic constituents closely.

[0016] The Applicant's operation of a semi-chemical (as well as any other chemical process) pulping recovery process can result in HAPs and toxic pollutants in the air, water, and solid waste. The primary cause for such toxic pollutant formation is the incomplete combustion of the SBL in the kiln. The fuel (lignin) contains primarily carbon, hydrogen, and oxygen atoms. When burned completely, the result is the formation of the maximum possible amount of heat (for recovery) and an air discharge consisting of water and carbon dioxide (minimal amount of air pollutants). When burned incompletely, the lignin combustion forms numerous toxic species and HAPs including naphthalene, benzene, methyl ethyl ketone, ethanol, and polycyclic aromatic compounds (PACs). The lighter toxic species (benzene, naphthalene, ethanol etc) are released to the ambient air as part of the combustion gas. The heavier toxic species (PACs) have low vapor pressures and include anthracenes, pyrenes etc. PACs tend be collected in the scrubber water. The scrubber water is returned to the process and is subsequently discharged to the wastewater treatment plant (WWTP). PACs are not easily treated or decomposed by an aerobic WWTP, and will pass through to be concentrated in the sludge. High concentrations of PACs in the sludge can result in the sludge becoming a hazardous waste and/or not meeting land application standards.

[0017] The result of incomplete SBL combustion is the inability to meet EPA MACT II standards, state-specific toxic air pollutant standards, and high liability and disposal costs from high PACs in sludge. Incomplete combustion also results in lower chemical recovery efficiencies and higher wear and tear on the kiln and associated equipment. The key to eliminating the above problems is to assure the complete combustion of the SBL.

[0018] In summary, there is a continuing need to improve the efficiency and lower the cost of the sodium recovery from spent liquor in a semi-chemical pulp digesting process.

SUMMARY OF THE INVENTION

[0019] The Applicant has developed a new method and apparatus for combusting black liquor generated by the semi-chemical pulping process. The invention consists of using a near-horizontal rotating kiln or furnace through which a draft airflow is maintained, and from which the soda ash is deposited out of the burning black liquor and collected through a lower end chute by action of the rotating motion of the furnace. The invention departs from the prior art at this point. Nozzle combustion air and black liquor are pumped into the furnace through a specifically designed mixing nozzle that is located within the lower, front end of the rotating portion of the furnace. A combustion aid such as fuel oil is added to the black liquor to promote ignition and permit a closely controlled combustion plume or flame adjacent the nozzle. The process is monitored and controlled to assure that, in accordance with the goals of the invention, combustion is complete and well below environmental air emission standards. Chemical recovery is efficient, clean, and free from products of incomplete combustion. Applicable U.S. governmental standards for air pollution, water, and solid waste are easily attained.

[0020] Development of the invention progressed through a stage at which strong spent black liquor at about a 50% solids ratio from a semi-chemical pulp mill is injected at the midpoint in length of a sloped, rotating furnace. A forced draft primary airflow fan provides the primary airflow to the lower or ash discharge end of the rotating furnace. An oil-fired air heater heats the primary air flow and maintains the furnace at combustion temperature. An induced draft fan at the exhaust or uphill end of the furnace directs exhaust airflow from the furnace to a heat recovery steam generator. The gas stream is then forced through a venturi scrubber and wet electrostatic precipitator and discharged to atmosphere.

[0021] The injected black liquor coats the rotating interior wall surface of the furnace near and downstream of the point of injection. Remaining water is evaporated. Baking then takes place as the solids continue to move downward with furnace rotation towards the ash discharge end of furnace. The organic component (dissolved wood) ignites and is incinerated on the furnace wall. The remaining ash is composed primarily of sodium compounds. The ash tumbles down the furnace and discharges into a dissolving tank. This solution is ready for re-use in the pulping process. The chemical recovery rate is about 70% of pulping usage. Furnace wall maintenance costs remain substantial, compared to prior methods.

[0022] In a significant further advance in the methodology employed for using the rotating kiln, in accordance with the invention, the system is reconfigured for direct, lower end, axial injection of atomized or vaporized black liquor ready for immediate ignition and supercharged burning to completion with primary air delivered at the nozzle, the combustion occurring substantially in the combustion plume in the immediate vicinity of the point of injection or nozzle end.

[0023] More particularly, a large volume of pre-heated low pressure secondary airflow is inserted at the lower end of the furnace, causing a positive pressure and a strong draft the length of the furnace. Steam atomized black liquor is injected through a nozzle or nozzles of a specially constructed SBL gun directly into the lower end of the rotating furnace, into the pre-heated airflow. A high pressure flow of primary air for nozzle combustion is mixed with the flow of vaporized liquor at the nozzle tip, promoting ignition of the black liquor and permitting an intense, controlled tip burn. The secondary airflow expands and supports a very large, intensely hot combustion plume extending several yards forward into the rotating furnace and laterally from the nozzle end towards the rotating walls of the furnace. The SBL gun is aptly referred to as the “Dragon” for its supercharged performance in the secondary airflow of the furnace.

[0024] Fuel oil, natural gas, or other more easily ignited material that acts as a combustion support fluid (CSF) may be continuously blended with the black liquor or supplied to the nozzle tip for injection flow blending to assist in initial ignition and burning consistency. The volume of combustion support fluid may be decreased or stopped as a stable burn condition is established, depending on the quality of the SBL and other process conditions.

[0025] Sensors monitor various process parameters affecting the operation, and may include the air and steam temperature, pressure and flow; the liquor pressure and flow; furnace and dissolving tank temperatures; nozzle performance and burn conditions; as well as exhaust data and gaseous emissions levels.

[0026] Exhaust gases are passed out the upper end of the furnace and processed as before. The ash and smelt resulting from the nozzle tip burn phenomenon is tumbled by the rotating furnace action back down into the existing dissolving tank as before. The methodology of the invention provides a more complete combustion of the combustible components of the black liquor in a substantially airborne circumstance rather than on the furnace wall, with recovery of the sodium carbonate ash accomplished as before, and allows the mill to meet more stringent federally mandated clean air requirements. It also exposes a significantly lower length of the furnace refractory wall to the highest intensity of the process, resulting in less burdensome maintenance program.

[0027] It is therefore among the goals of the invention: to ensure complete combustion of SBL as measured by exhaust levels of carbon monoxide and methane below 20 ppmv; to ensure that exhaust gas emissions are well below 2.97-lb THC/TBLS; to ensure all other toxic emissions in the exhaust gas and in the scrubber water area are very low to non-detectable by industry standards; to increase potential burner capacity by 50% over; to allow for auxiliary fuel injection to ensure consistent fuel quality to the burner; and to provide for smelt ring control and reduced kiln brick erosion. Yet further goals include: dropping PAC levels in the WWTP sludge to negligible levels; increasing steam generation rates from the waste heat recovery boiler; and improving kiln combustion control dynamics to permit full control automation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028]FIG. 1 is a block diagram of the kiln and associated principle components of the preferred embodiment system.

[0029]FIG. 2 is a longitudinal cross section view of a preferred embodiment burner and nozzle assembly of the FIG. 1 embodiment.

[0030]FIG. 3 is a simplified process diagram of a preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0031] The invention is susceptible of many embodiments. A preferred embodiment, including preferred operating ranges of some parameters, is described as follows. A large, rotatable kiln typically has a diameter of 4 to 14 feet, and a length of 40 to 200 feet, and is oriented at a slight angle off horizontal such that rotation will cause loose material deposited within the kiln to be carried gradually towards the lower end of the kiln. Mechanical rotation of the kiln is provided at speeds of 0.2 to 3.0 rpm. The kiln is fire brick lined with appropriate temperature shielding components. There is an induced draft fan at the exhaust end of the kiln.

[0032] The combustion system of the invention is oriented in a non-rotatable configuration at the lower end of the kiln. There is a forced draft fan for providing primary airflow through the kiln, and a standard oil or natural gas fired burner for heating this primary airflow for preheating the kiln.

[0033] More particularly, there is in accordance with the invention a black liquor burner assembly or gun which may incorporate: a burner barrel with conical end; swirled high pressure, high velocity ignition air injection; nozzle end swirler for blending the air/fuel mixture; steam atomizer; black liquor gun and spray nozzle; atomizer plates; spark rod; gas supply pipe; gas igniter assembly; and flame detection sensors and burner management controls.

[0034] Forced ignition primary air provides flame stability by injection through the swirler. The ratio of total airflow through the furnace to swirled primary airflow in the burner assembly is manually and automatically adjustable, and is preferably maintained at an approximate ratio of 3:1. One skilled in the art will appreciate that the significance difference in bore sizes means that the primary airflow is a higher pressure, higher velocity airflow than the secondary or furnace draft airflow, resulting in a supercharged or torch-like effect. The burner assembly is mounted on a fixed or non-rotating base support located at the lower end of kiln, so that the nozzle end protrudes 5-15 feet into the rotating portion of the kiln.

[0035] The black liquor burning capacity of the preferred embodiment is 10 to 30 gpm (gallons per minute) SBL. The burner assembly is configured to provide a combustion support fluid flow rate of up to 2 gpm for blending with the black liquor. The preferred fluid is fuel oil.

[0036] Once the process is understood, automated controls such as are known in the art may be configured and utilized for steam and SBL fluid flow handling, airflow balancing, ignition, rate of combustion support fluid flow, and ratio of steam to SBL differential atomization pressure.

[0037] With respect to the supply system, the SBL is preheated by a heat exchanger to a minimum of 200 F. The atomizer dry steam pressure is maintained at 10-30 psig. Variable positive displacement SBL pumps deliver up to 30 gpm of SBL at 140 psig at the burner. The SBL flow is measured by a flowmeter. Pressure gauges measure steam and SBL pressures at the heat exchanger, pumps, and burner. The system incorporates SBL back flush fittings and valves.

[0038] The invention is susceptible of various method and apparatus embodiments. What follows is merely exemplary, based on an existing kiln modified for the purposes of the invention. It is presented as a preferred embodiment, as the capital costs associated with such systems are very significant and it is likely that existing rotating kilns can be modified, as in the Applicant's case, much more quickly and economically than new equipment can be brought on line.

[0039] Referring to FIG. 1, there is a prior art rotating furnace 10, about 200 feet long and 12 feet in diameter, arranged at a slight angle of a few degrees off horizontal so that particulate matter precipitated out of the combustion process will be tumbled by rotation from any point along the length of the furnace around the wall and towards the lower end of the furnace. There is a draft air supply system 20 connected to the lower end of furnace 10 for supplying a pre-heated, low pressure, high volume airflow through the furnace from the low end to the high end. It can achieve discharge air temperatures of about 1000 degrees when the kiln is in full operation.

[0040] Specific to the invention, there is an SBL burner/nozzle assembly 100, about 22 feet long, piercing the lower end wall of furnace 10 and protruding into the rotating region of the kiln about ten feet, the nozzle end configured to dispense, ignite, and burn steam atomized black liquor in a high velocity primary airflow. SBL burner/nozzle assembly 100 is provided with an igniter and three inputs; preheated, steam atomized SBL; a combustion support fluid (CSF) such as fuel oil; and primary combustion air (PRIMARY AIR) for nozzle combustion air flow.

[0041] As in the prior art, there is a discharge chute at the lower end of furnace 10 connected to a dissolver tank 30 where a sodium recovery process is initiated. The upper or gas discharge end of furnace 10 is connected to a heat recovery steam generator 40 through which hot exhaust air is drawn by induced draft fan 50. From there the exhaust gases are pushed on through scrubber & wet electrostatic precipitator (WESP) processing equipment 60 and is eventually exhausted to the environment.

[0042] Referring to FIG. 2, a burner/nozzle assembly 100 of FIG. 1, also referred to as the SBL “gun”, is illustrated in more detail. Burner barrel 102 has a nozzle end terminating in a cone extension 122 and an aft end with a cover plate 104, through which the aft end of gun pipe 112 protrudes. A right angle side branch air feed 106 connects a high velocity, nozzle air flow supply to barrel 102. Gun pipe 112 extends through the center of burner barrel 102 to the nozzle end of the barrel, discharging steam atomized SBL into the nozzle air flow provided by branch air feed 106 through the rotating blades of swirler 124. A spark rod 107 and a combustion support fluid supply line 108, supplying fuel oil in the preferred embodiment, also extend through burner barrel 102, terminating at the nozzle end of the gun and providing control over the ignition/combustion process at the nozzle. Swirler 124 is semi-enclosed by cone extension 122 to help contain and shape the combustion plume.

[0043] Referring now to FIG. 3, there is depicted a simplified process diagram for the preferred embodiment, in which the process is conducted as follows. Within a rotating furnace suitably configured with an SBL mixing gun of the invention: steam atomized SBL is supplied to the Mixture; combustion support fluid (CSF) is combined with the SBL as needed, being either or both blended with liquid or steam atomized SBL or introduced directly into the Mixture to provide reliable ignition and consistent heating value; high velocity primary combustion air (Primary Air) is supplied to the Mixture; Ignition is triggered within the Mixture, causing Combustion; Draft Air further supports and expands the Combustion; out of which Ash is precipitated, which is then passed into a Sodium Recovery process; and Gas is exhausted from the furnace, passed through a Heat Recovery system and a Scrubber system, and then Discharged to atmosphere. Suitable process controls are applied to the apparatus within which the process is conducted, to insure optimal burn conditions and complete combustion.

[0044] Other embodiments and variations of the system are within the scope of the invention. For example, the rotating kiln or rotating portion of the kiln may be much shorter than in the existing kiln of the present embodiment, with or without scaling, as the necessary length at the same scale is less than 100 feet, preferably less than 50 feet. The angle of slope of the furnace or kiln, and the speed of rotation can be optimized to collect the hot ash and carry it to the lower end discharge chute at a rate consistent with the volume of black liquor being consumed in the burn.

[0045] The nozzle assembly may be more than one assembly or the assembly may have more than one nozzle or point of dispersement. The nozzle may be, for example, a ring nozzle assembly with atomized black liquor being sprayed from several nozzles at several angles into a hot, high velocity primary air flow with suitable control of pressures and flow so as to enable adequate air/fuel mixing, close control and precise burn conditions, while a balance of primary to total airflow is used to maintain a combustion plume scaled to the size of the kiln, exhaust capacity, and the speed of recovery of the sodium ash.

[0046] Other embodiments are within the scope of the invention. For example, there is a method and apparatus for combustion of black liquor and recovery of sodium carbonate ash consisting of the steps of using a near-horizontal rotating furnace with a mixing nozzle for black liquor and combustion air configured within its lower end, directing a preheated airflow through the furnace from lower to upper end, pumping black liquor and combustion air through the mixing nozzle so as to spray a combustible mixture of air and black liquor from the nozzle, igniting the mixture of air and black liquor so as to cause a combustion plume extending outward from the nozzle, controlling the process conditions of the combustion plume, emitting exhaust gases out the upper end of the rotating kiln, and recovering sodium carbonate ash through a chute at the lower end of the furnace.

[0047] There may also be a mixing of a combustion support fluid or agent with the black liquor to support ignition and control combustion, process instrumentation for the monitoring of the process conditions of the combustion plume or burn, and controls for controlling the relative and total volumes, temperatures and pressures of black liquor, steam, combustion agent, airflows, and airflow balance.

[0048] There are methods within the scope of the invention, including a method for combustion of black liquor and collection of sodium carbonate ash consisting of the steps of using a near-horizontal rotating furnace configured with a lower end source of draft airflow, a lower end mixing nozzle assembly, a lower end chute for collection of sodium carbonate ash, and an upper end air mover of exhaust gases; then directing a preheated draft airflow forward through the furnace from lower to upper end, spraying a combustible mixture of strong black liquor and high velocity combustion airflow forward into the furnace with the mixing nozzle assembly, and igniting the combustible mixture proximate the nozzle so as to cause a combustion plume extending outward from the mixing nozzle into the draft airflow. Exhaust gases are emitted at the upper end of the furnace for further processing, and sodium carbonate ash is collected on the walls of the furnace and hence through the lower end chute.

[0049] The strong black liquor may be steam atomized strong black liquor. It may have an added combustion agent such as fuel oil. The combustion agent may be added to the strong black liquor and thereafter be steam atomized, and/or it may be injected directly into the mixture.

[0050] The methods may employ steps of controlling the ratio of combustion agent to strong black liquor for uniform burning quality, controlling the ratio of total airflow to combustion airflow, maintaining preferably a ratio at about 3:1. The control steps may include controlling the steam to strong black liquor differential atomization pressure of the steam atomized strong black liquor.

[0051] Another embodiment within the scope of the invention is a system for combustion of black liquor and collection of sodium carbonate ash consisting of a furnace with a lower end and an upper end; a lower end source of draft airflow; a lower end mixing nozzle assembly; a lower end chute for collection of sodium carbonate ash; an upper end air mover of exhaust gases; and a process control system. The lower end source of draft airflow is configured for directing a preheated draft airflow through the furnace from lower to upper end. The mixing nozzle assembly is configured for spraying a combustible mixture of steam atomized strong black liquor, combustion agent, and high velocity combustion airflow into the furnace. The mixing nozzle assembly is further configured with means such as an igniter pipe or electronic ignition for igniting the combustible mixture proximate the nozzle so as to cause a combustion plume extending outward from the mixing nozzle into the draft airflow.

[0052] The mixing nozzle assembly may consist of a burner barrel with an open nozzle end and an aft end closed by a cover plate; a gun pipe within the burner barrel extending the length thereof, with one end protruding through the cover plate and connected to a source of steam atomized strong black liquor, and the other end being open and terminating at the open nozzle end of said burner barrel. There is a side branch combustion airflow air feed connecting to the burner barrel so as to direct combustion airflow along the gun pipe and out the nozzle end. There is a swirler or other such rotatable or undulating or vaned mixing device mounted at the nozzle end at which the gun pipe and the combustion airflow are directed. And there is an igniter pipe or device configured for applying an igniting event proximate the open nozzle end.

[0053] There may be on the mixing nozzle assembly a feed line or pipe for a combustion agent running within the burner barrel, with one end of the feed line protruding through the aft end cover plate and connected to a source of combustion agent, and the other end of the line or pipe being terminated with a combustion agent nozzle proximate the nozzle end of the burner barrel and directed so as to mix combustion agent with the mixture of SBL and combustion air. Also, the burner barrel may have a cone extension or other terminal structural feature at the nozzle end for containing and shaping a combustion plume.

[0054] The furnace may be a near-horizontal rotating kiln furnace configured with an upper end exhaust gas fan. Furnaces of vertical or other orientations are likewise adaptable to the method and apparatus of the invention. Also, as is anticipated in most cases, the upper end or gas discharge end of the furnace is likely to be connected to a scrubber and wet electrostatic precipitator processing system, or other heat recovery and air quality treatment systems, and the lower end chute will be connected to a sodium recovery processing system.

[0055] Embodiments of the invention will likely incorporate a process control system consisting of a operator interface to a computer system connected to suitable sensors, actuators, pumps, and valve means for controlling the igniter, and all or some of other parameters such as the ratio of combustion agent to strong black liquor for uniform burning quality, the ratio of total airflow to combustion airflow and the steam to strong black liquor differential atomization pressure of the steam atomized strong black liquor.

[0056] The scope of the invention is not limited to the preferred embodiments which are merely examplary, but will be clear to those skilled in the art from the description, figures and claims that follow. 

We claim:
 1. A method for combustion of black liquor and collection of sodium carbonate ash comprising the steps: using a near-horizontal rotating furnace configured with a lower end source of draft airflow, a lower end mixing nozzle assembly, a lower end chute for collection of sodium carbonate ash, and an upper end air mover of exhaust gases; directing preheated said draft airflow forward through said furnace from lower to upper end, spraying a combustible mixture of strong black liquor and high velocity combustion airflow forward into said furnace with said mixing nozzle assembly, igniting said combustible mixture proximate said nozzle so as to cause a combustion plume extending outward from said mixing nozzle into said draft airflow, emitting exhaust gases at the upper end of said furnace, and collecting said sodium carbonate ash on the walls of said furnace and hence through said lower end chute.
 2. A method according to claim 1, said strong black liquor being steam atomized strong black liquor.
 3. A method according to claim 2, said steam atomized strong black liquor having an added combustion agent.
 4. A method according to claim 2, further comprising the step: said steam atomized strong black liquor comprising a combustion agent added to strong black liquor and thereafter being steam atomized.
 5. A method according to claim 2, further comprising the step; injecting a combustion agent into said mixture.
 6. A method according to claim 3, further comprising the step: controlling the ratio of combustion agent to strong black liquor for uniform burning quality.
 7. A method according to claim 3, further comprising the step: controlling the ratio of total airflow to said combustion airflow.
 8. A method according to claim 7, said step of controlling the ratio comprising maintaining said ratio at about 3:1.
 9. A method according to claim 2, further comprising the step: controlling the steam to strong black liquor differential atomization pressure of said steam atomized strong black liquor.
 10. A system for combustion of black liquor and collection of sodium carbonate ash comprising: a furnace with a lower end and an upper end; a lower end source of draft airflow; a lower end mixing nozzle assembly; a lower end chute for collection of sodium carbonate ash; an upper end air mover of exhaust gases; and a process control system; said lower end source of draft airflow configured for directing a preheated said draft airflow through said furnace from lower to upper end, said mixing nozzle assembly configured for spraying a combustible mixture of steam atomized strong black liquor, combustion agent, and high velocity combustion airflow into said furnace, said mixing nozzle assembly further configured with means for igniting said combustible mixture proximate said nozzle so as to cause a combustion plume extending outward from said mixing nozzle into said draft airflow.
 11. A system according to claim 10, said mixing nozzle assembly comprising a burner barrel with an open nozzle end and an aft end closed by a cover plate; a gun pipe within said burner barrel extending the length thereof, one end protruding through said cover plate and connected to a source of steam atomized strong black liquor, the other end being open and terminating at said open nozzle end of said burner barrel; a side branch combustion airflow air feed connecting to said burner barrel so as to direct combustion airflow along said gun pipe and out said nozzle end; a swirler mounted at said nozzle end at which said gun pipe and said combustion airflow are directed; and an igniter configured for applying an igniting event proximate said open nozzle end.
 12. A system according to claim 11, said mixing nozzle assembly further comprising a feed line for a combustion agent within said burner barrel, one end of said feed line protruding through said cover plate and connected to a source of said combustion agent, the other end terminating in a combustion agent nozzle proximate said nozzle end of said burner barrel.
 13. A system according to claim 12, said burner barrel further configured with a cone extension at said nozzle end for containing and shaping a combustion plume.
 14. A system according to claim 11, said furnace comprising a near-horizontal rotating kiln furnace configured with an upper end exhaust gas fan.
 15. A system according to claim 11, said upper end of said furnace connected to a scrubber and wet electrostatic precipitator processing system, said lower end chute connecting to a sodium recovery processing system.
 16. A system according to claim 10, said process control system comprising computer, sensors, actuators, pumps, and valve means for controlling the ratio of combustion agent to strong black liquor for uniform burning quality.
 17. A system according to claim 10, said process control system comprising computer, sensors, actuators and valve means for controlling the ratio of total airflow to said combustion airflow.
 18. A system according to claim 10, said process control system comprising computer, sensors, actuators and valve means for controlling the ratio comprising maintaining said ratio at about 3:1.
 19. A system according to claim 10, said process control system comprising computer, sensors, actuators, pumps and valve means for controlling the steam to strong black liquor differential atomization pressure of said steam atomized strong black liquor.
 20. A system for combustion of black liquor and collection of sodium carbonate ash comprising: a furnace with a lower end and an upper end; a lower end source of draft airflow configured for directing a preheated said draft airflow through said furnace from lower to upper end; a lower end mixing nozzle assembly comprising a burner barrel with an open nozzle end and an aft end closed by a cover plate, a gun pipe within said burner barrel extending the length thereof, one end protruding through said cover plate and connected to a source of steam atomized strong black liquor, the other end being open and terminating at said open nozzle end of said burner barrel, a side branch combustion airflow air feed connecting to said burner barrel so as to direct combustion airflow along said gun pipe and out said nozzle end, a swirler mounted at said nozzle end at which said gun pipe and said combustion airflow are directed, and an igniter configured for applying an igniting event proximate said open nozzle end; a lower end chute for collection of sodium carbonate ash; an upper end air mover of exhaust gases; and a process control system comprising computer, sensors, actuators, pumps, and valve means for controlling said igniter, the ratio of combustion agent to strong black liquor for uniform burning quality, the ratio of total airflow to said combustion airflow and the steam to strong black liquor differential atomization pressure of said steam atomized strong black liquor. 